Just stumbled on this post now (always late to the party).
I have both a 900F and an 1100F (had the 900 since new, 1100 more recently acquired).
Generally the F's are relatively reliable and can take some beating but there are a few known weakness and issues.
It's not the cams that are the problem (although they are generally thought to be made of soft steel so they can wear out however probably not really an issue if you’re not racing it- and as with most bikes, oil changes are critical); the problem is with the cam chains - they can snap which of course leads to all sorts of problems and expense. To compound things, the newer superseded cam chains are not as strong as the originals. Believe it or not, lots of folks leave the original cam chains in when doing a full rebuild. The crappy new cam chain problem has been somewhat mitigated recently by the discovery that a Yamaha Raptor apparently uses the correct size, but much stronger cam chain. In addition, there's a fella in Auz that makes replacement cam chain tensioners and guides that are supposed to be a vast improvement over originals (which are no longer available) and which basically eliminate the faulty chain problem; personally, I think the jury is still out on their effectiveness (just because it too early to sell for sure). If you found a bike that wasn't beaten, and you don't beat it, you're probably fine.
Adjusting the valves is critical. Most folks think the factory recommendation (two thou, I think) is way too tight and should be more like 5~6 thou. Misadjusted valves will display all manner of strange starting and running behavior. In addition if the valves are to tight, they can burn which is an expensive problem to rectify, even if you’re doing it yourself.
There's a problem with the clutch: the dampers in the clutch basket are made of rubber. When they get old, they shrink a bit and become hard which causes the basket to knock around a bit. Not really a problem, but you'll find the bike will probably make a strange rattle sound at idle with clutch out, pull in the clutch and the noise disappears. They all do it. There's a guy in Holland that sells a kit to replace the rubbers with new ones which eliminate the rattle.
Similar rubber dampers are in the primary shaft (or secondary shaft, or some shaft) and they too will shrink and become hard and make very embarrassing knocking sounds when cold. Not a deal breaker unless you’re doing major surgery anyway - replacements used to be available from Honda but recently maybe not?? You'd need to split the crankcase to put them in.
Charging system can cause problems: the F’s have a true alternator with an excited field rotor. All that high RPM spinning can cause the wires in the rotor to chaff, and lead to a short circuit, which if left long enough, will take out the R/R due to overheating trying to pump more current into the faulty rotor. Easy fix, but could leave you stranded. There are some easy common measurements to make to help ensure things are working as they should (but measurements not infallible due to high speed nature of spinning rotor). Aftermarket and rewound rotors are available in addition to R/R’s.
Finally, the quality of the original valve seals in these very hot running engines was really crappy and most of these older bikes with original seals burn a lot of oil. Fortunately, newer Viton seals completely eliminate the problem, and if you’re creative, you can change them without removing the head, but you will probably have to remove the motor from the frame (although some have even got around that). Unless you’ve found a very low mileage model, and even then, you’ll probably want at some point to change the valve seals.
The one mechanical item that will prevent perpetual rebuilding of these bikes is the oil pumps. No longer available from Honda and even good used ones are almost impossible to find. Probably not an issue for you but maybe for some potential rebuilder down the road.
Lots of these bikes were sold so there's plenty of used parts out there - EBay is your friend. Not much in the way of aftermarket parts with the exception of oil and air filters and generic parts like bars, tires, etc.
These bikes still provide a spirited and fun ride but are outmatched on every scale by newer bikes (some diehards would disagree with that point of view, of course) with the exception of looks - In case I didn’t mention it, I think these F bikes are among the prettiest naked bikes ever built (completely biased opinion).
If your jurisdiction is like mine, and if the bike is in good shape, you may be eligible for 'collector’ insurance which makes riding these old bikes very economical.
That's about it: change the oil, adjust the valves, ride.
There's a very active forum with huge amounts of good advice, technical information and lots of involved enthusiasts:
http://www.cb750f.com/ Hope this helps.
PS: Forgot one big thing: the starter clutches are finicky and prone to fail. They give you a warning before they die (and you can always bump start) and since parts are available and the repair is relatively simple, it's no big deal really. Recently some guy has adapted a Yamaha R6 (I think) starter clutch to fit which permanently eliminates the problem (although again, that solution is new so AFAIC, the jury is still out a bit).
